1. Field of the Invention
The present invention relates to telephone systems that implement group calls by using time-division voice exchange and may be used for in-facility radio systems. Particularly, the invention relates to a telephone system that can synthesize voice efficiently and at low costs flexibly in accordance with multiplexing or demultiplexing of plural group calls and can control the voice level and/or voice quality of a multiplexed group call in an optimum way.
2. Description of the Related Art
[Description of the Related Art/FIG. 26]
Hitherto, there have been telephone systems that implement group calls by using time-division voice exchange. With reference to FIG. 26, a conventional telephone system will be described. FIG. 26 is a schematic configuration diagram of a conventional telephone system.
As shown in FIG. 26, a conventional telephone system includes a control unit 1′, a time switch 2, conference trunks 3-1 to 3-n, a line interface unit 4, voice message output means 6 and a state indicating unit 7 fundamentally. The line interface unit 4 further connects to a base station 51, a telephone 52, a contact input/output unit 53 and a dedicated line etc. 54 and to a mobile device 55 through the base station 51.
Voice highways through which voice signals are transmitted are provided between the time switch 2 and the conference trunk 3, line interface unit 4 and voice message output means 6 and between the line interface 4 and the base station 51, telephone 52, contact input/output unit 53 and dedicated line etc. 54.
[Configurations of Components]
The components will be described more specifically.
The control unit 1′ may be a computer including a microprocessor, peripheral circuits and a memory, for example, and starts a program stored in the memory and controls the time switch 2, conference trunk 3 and line interface 4 so as to perform processing for implementing a private call or group calls.
The time switch 2 connects to an input highway and output highway through which a voice signal are transmitted and is a switch that performs time-division exchange for each time slot of the highways. The time switch 2 can exchange (or connect) a time slot of an arbitrary highway with (or to) a time slot of another highway in accordance with the instruction from the control unit 1′.
The conference trunk 3 performs voice synthesis processing including synthesizing and outputting voice of plural callers and synthesizes voice data (which is PCM-encoded data) of plural input time slots and outputs the result to a time slot of one output highway. Thus, a group call state can be acquired. The number of time slots of an input highway of a conventional conference trunk is 32, whereby 32 people can participate in a conference.
In the conventional system, each of the conference trunks 3-1 to 3-n is configured by an independent LSI chip.
The line interface unit 4 connects between the base station 51, telephone 52, contact input/output 53, dedicated lines etc. 54 and so on and the time switch 2 and control unit 1′. Control signals and a voice highway are connected, and the control signals are connected to the control unit 1′, whereby call control signals such as ON hook, OFF hook, dialing, a contact input event and a contact output event are transmitted. The voice highway connects to the time switch 2 and a voice signal (PCM code) is transmitted by the voice highway.
The voice message output means 6 includes a memory storing voice message data recorded in advance and a circuit that outputs a voice message to a time slot of a highway. Examples of the message may be a message “The terminal you are calling is not available because it is outside of its service area or is switched off”.
The state indicating unit 7 connects to the control unit 1′ and is a personal computer including a display and an operation unit. The state indicating unit 7 is a maintenance terminal that indicates the state of the control unit 1′ or maintains the system.
[Conventional Group Calls/FIG. 27]
In the conventional telephone system the control unit 1′ prestores a group call special number for participating in a group call and members who perform the group call and associates the group call special number and the conference trunk 3 that performs the voice synthesis.
If the group call special number is called, the control unit 1′ calls the members corresponding to the group call special number and connects the caller of the special number and the OFF hooking members to the conference trunk 3 corresponding to the group call special number to provide the group call.
With reference to FIG. 27, voice synthesis in the group call will be described. FIG. 27 is a schematic explanatory diagram of a voice highway for a conventional group call.
As shown in FIG. 27, when members A, B and C are performing a group call, the member A connects to an input highway a and an output highway a, the member B connects to an input highway b and an output highway b, and the member C connect to an input highway c and an output highway c. The input highways are used to transfer voice, and the output highways are used to transfer receive voice.
The voice data on the input highways a, b and c input from the line interface unit 4 to the time switch 2 are output from the time switch 2 through the output highways to the specific conference trunk n in accordance with the instruction from the control unit 1′.
The conference trunk n outputs the data resulting from the synthesis of voices of B and C to the time slot corresponding to A of the input highway d.
Similarly, the conference trunk n outputs the data resulting from the synthesis of voices of A and C to the time slot corresponding to B of the input highway d and outputs the data resulting from the synthesis of voices of A and B to the time slot corresponding to C of the input highway d.
The time switch 2 outputs the time slots to the corresponding output highways a, b and c.
Thus, each of the members can hear all voices of the other members excluding the voice of himself/herself, implementing the group call.
[Talking Together Among Plural Group Calls/FIG. 28]
In the conventional telephone system, the members are fixed by a group call special number. Thus, in order to implement both of the conference with a specific group call and the conference including the members of another group call, group special numbers, members and conference trunks must be defined in association for both of a group call for a small number of people and a group call for a large number of people.
Talking together among plural group calls will be described with reference to FIG. 28. FIG. 28 is a schematic explanatory diagram showing a method for talking together among plural group calls in the conventional telephone system.
As illustrated in FIG. 28, the conventional telephone system may have definitions of, among members 1 to 8, a group call A of 1 to 4 (“conference call A” in FIG. 28), a group call B (conference call B) of 5 to 8 and a group call C (conference call C) of 1 to 8.
After the group call A and group call B start, if it is determined to start a conference in conjunction between the group A and group B, the members of the group call A and the members of the group call B once leaves the group calls. After that, one of the members calls the group call special number C newly to start the group call C including the members 1 to 8. This complicates the terminal operations.
[Multiplexing Plural Group Calls in Conventional Telephone System]
There has been another method that couples plural group calls on voice lines into an appearance of one group call. For example, according to the method, the voice of a group call A is fed onto an analog line A, the voice of a group call B is fed onto an analog line B, and the analog lines A and B are physically connected. Thus, the plural group calls can be multiplexed.
Also on digital lines, the synthesized voice of two group calls may be further synthesized by using another conference trunk so that the plural group calls can be multiplexed. (Refer to JP-A-2002-218068).
Generally, when voices of plural lines are synthesized, the voice level is increased (or is saturated), which causes a distortion and difficulty in hearing. In order to solve the problem, each conference trunk lowers the input voice level before the synthesis to perform the level control in the synthesis for an optimum level.
However, in the conventional telephone system, the method for the apparent multiplexing of plural group calls includes controlling the voice level and/or voice quality at the conference trunks before the multiplexing. For that reason, it is difficult to control the voice level and/or voice quality in consideration of all voices after the multiplexing, and it is thus difficult to adjust the level and voice quality in an optimum way, which is a problem.
Furthermore, in the conventional telephone system, there is no function to notify whether the state of the target group call to be multiplexed or demultiplexed is currently a result of multiplexing or demultiplexing, which is inconvenient for an operator or a maintenance operator and is another problem.